Recent evidence suggests that the antigen receptor(s) of T-cells bear immunologically defined determinants that cross-react with idiotypic determinants on antibody. If this is true then anti-idiotypic sera provide us with a tool for the analysis of such receptors. (T,G)-A--L is a synthetic polypeptide antigen, the response to which is under H-2 linked Ir gene control. Antisera have been raised against idiotypic determinants of anti-(T,G)-A--L antibody. Data concerning the specificity of these reagents and a preliminary analysis of the genetic control of idiotype expression on antibody are shown indicating that the anti-idiotypic antisera can inhibit (T,G)-A--L specific cellular function; and that the most likely locus of this effect is on T-cells. On the basis of these results, a series of experiments to explore the genetic control and structural basis of idiotype expression on T-cells is proposed. The initial objective will be to establish the genetic control of idiotype expression on anti-(T,G)-A--L antibody by surveying a number of inbred mouse strains, F1 and backcross animals. A radioimmunoassay will be used to detect idiotypes expressed on antibody. The second objective is to study the genetic control of idiotype expression on (T,G)-A--L specific T-cells using antigen specific T-cell proliferation to measure effects of the antiidiotypic antisera on cellular function. The last objective is to develop a method whereby the molecular aspects of T-cell derived idiotype bearing structures can be studied. By internally or externally labeling (T,G)-A--L specific blast cells and using the anti-idiotypic sera to immunoprecipitate idiotype bearing structures, such a objective may be accomplished.